Sometimes the properties of subsurface materials, such as soil, liquids, gasses or chemicals, are determined by bringing samples of the material to the surface for inspection and measurement. Sometimes it is preferred to obtain such information in situ, that is, underground where the material is found. Information can be gathered from sensors, for example, as the sensors are pushed through the soil, to obtain a log or chart of information as a function of depth or position.
Cone Penetrometer Testing (CPT) is one known method of obtaining logs of subsurface material properties. In general terms, CPT features a cone-tipped probe pushed in a non-rotary sense through the soil, typically in a vertical direction. Sensors in the probe measure such properties as bearing load and soil friction, from which various other properties of the soil can be inferred. Other sensors are also included in the probe, such as for measuring subsurface pore fluid pressure. Various other sensors have also been developed for deployment by CPT methods. In environmentally sensitive sites, direct push techniques like CPT are preferred because the relatively small holes they leave in soil can be filled with grout as the probes are withdrawn, and do not generate the volume of tailings produced by drill-sampling methods.
The probe depths obtainable by direct push methods like CPT are limited, primarily due to limits on the amount of push force that can be applied. While depths of 100 meters or more have been obtained in some geologies, depths of only about 50 meters are more common, as soil friction on the outer surface of the probe and push rods eventually exceeds the amount of downward force that can be safely applied at the surface. In other cases, an immovable obstruction, such as bedrock, is encountered. Either event is called “refusal” and represents the extent of the depth obtainable for that push.